Bump version.log4j from 2.12.4 to 2.21.0

Bumps `version.log4j` from 2.12.4 to 2.21.0.

Updates `org.apache.logging.log4j:log4j-core` from 2.12.4 to 2.21.0

Updates `org.apache.logging.log4j:log4j-slf4j-impl` from 2.12.4 to 2.21.0

---
updated-dependencies:
- dependency-name: org.apache.logging.log4j:log4j-core
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.apache.logging.log4j:log4j-slf4j-impl
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

apm-agent-plugins/apm-ecs-logging-plugin/pom.xml

@@ -48,7 +48,7 @@
         <dependency>
             <groupId>org.apache.logging.log4j</groupId>
             <artifactId>log4j-core</artifactId>
-            <version>2.20.0</version>
+            <version>2.21.0</version>
             <scope>provided</scope>
             <optional>true</optional>
         </dependency>

apm-agent-plugins/apm-logging-plugin/apm-log4j2-plugin/pom.xml

@@ -24,7 +24,7 @@
         <dependency>
             <groupId>org.apache.logging.log4j</groupId>
             <artifactId>log4j-core</artifactId>
-            <version>2.17.1</version>
+            <version>2.21.0</version>
             <scope>provided</scope>
         </dependency>
         <dependency>

pom.xml

@@ -125,7 +125,7 @@
         <!-- this is the last version of log4j that is compatible with Java 7. Due to a known vulnerability
         (https://nvd.nist.gov/vuln/detail/CVE-2020-9488#vulnCurrentDescriptionTitle), the SMTP appender is
          excluded from the build and not packaged into the agent artifacts -->
-        <version.log4j>2.12.4</version.log4j>
+        <version.log4j>2.21.0</version.log4j>
         <version.ecs.logging>1.5.0</version.ecs.logging>
         <version.spring>5.0.15.RELEASE</version.spring>
         <version.jetty-server>9.4.11.v20180605</version.jetty-server>